An experimental animal model in which the course of immunodeficiency virus infection parallels the pathogenesis of the human disease is critical for the study of human AIDS. Simian immunodeficiency virus (SIV) infection of macaques satisfies this criterion and is therefore a relevant model. SIV induces an immunodeficiency syndrome in infected macaques that is remarkably similar in pathogenesis to human AIDS. An important use of this animal model system is the detailed study of pathogenesis and viral determinants of disease since many studies of this type are not feasible in humans. The purpose of this project is to investigate host and viral factors involved in variable disease progression in SIV-infected macaques and the lack of disease in African primates infected with their own strains of SIV. [unreadable] [unreadable] PATHOGENESIS OF SIVsm-INFECTION OF MACAQUES:[unreadable] To investigate the role of host factors in SIV-infection of macaques, we used a well-defined molecularly cloned virus (SIVsmE543-3). Our initial studies have focused on defining the immune and viral factors involved in rapid disease progression. This small subset of SIV-infected macaques fails to maintain SIV-specific immune responses and progress to AIDS in less than six months from the time of inoculation. SIVsmE543-3-infected RP macaques showed profound and irreversible depletion of memory CD4+ T cells that likely explain the immune deficits in these animals. Molecular studies of virus in tissues and plasma of rapid progressor macaques exhibited common substitutions in the env gene. These substitutions were unusual in that they involved residues that were generally conserved and that were known to affect binding of env to CD4 or coreceptor. Virus from RP macaques was molecularly and biologically characterized and full length infectious molecular clones of various RP viruses were derived from plasma of a RP macaque, H635. The H635 viruses replicated less efficiently in primary macaque PBMC and macrophages than the parental SIVsmE543-3 strain and were much more senstitive to neutralizing antibody. Macaques inoculated with one of these viruses, H635-FC developed moderate to high primary and setpoint viremia, and progressive CD4+ T cell depletion but did not develop rapid disease. Sequence analysis of plasma virus in these animals demonstrated rapid loss of RP-specific env mutations concurrent with the development of neutralizing antibody response. These studies suggested that rapid progressor mutations are the consequence, not the cause of rapid disease progression.[unreadable] [unreadable] In studying macaques inoculated with SIVsmE543-3 and H635FC, we observed two distinct patterns of CD4 loss, primarily affecting memory CD4+ T cells (Memory depleted or MD) and primarily affecting nave CD4+ T cells (nave depleted, ND). Most of the CD4+ T cell loss in MD macaques appears to be the direct result of virus-induced killing; however nave CD4 loss is harder to explain since SIV primarily uses CCR5 that is not expressed on nave CD4 T cells. ND macaques were distinguished from MD macaques, by intense immune activation and the presence of autoantibodies to a number of self-antigens, including T cell antigens. Studies are ongoing to examine the mechanisms of nave CD4 T cell loss in ND macaques and determine whether this depletion might be explained by autoimmune destruction. [unreadable] [unreadable] We have also evaluated the pathogenicity of the three different SIVagm isolates in pigtailed (PT) macaques. Infection of macaques with any of the three isolates resulted in high levels of primary plasma viremia by one week after inoculation. Viremia was quickly controlled following infection with SIVagm155; these animals have maintained CD4+ T cell subsets and remain healthy. The plateau levels among SIVagm90 and SIVagm9063-inoculated macaques varied widely from 100 to a million copies/ml of plasma. Three of four animals from each of these groups progressed to AIDS. Setpoint viremia and the degree of CD4+ T cell loss at six months post infection were not significantly different between macaques inoculated with SIVagm90 and SIVagm9063. However, these parameters were significantly different in SIVagm155-inoculated macaques (p values < 0.01). Considering all the macaques, the degree of CD4+ T cell loss by six months post infection, correlated with the plateau levels of viremia. Thus, similar to SIVsm/mac-infection of macaques and human AIDS, viral load is an excellent prognostic indicator of disease course.[unreadable] [unreadable] ASYMPTOMATIC INFECTION OF NATURAL HOST SPECIES[unreadable] A second goal of this project is to study the mechanisms underlying the lack of pathogenicity of SIV for their natural host species, with emphasis on SIVagm from vervet monkeys. SIVagm is capable of inducing AIDS in PT macaques but African green monkeys (AGM) do not develop overt signs of disease following infection. We evaluated the viral kinetics of a natural SIVagm isolate in two species of AGM, vervet monkeys (the species of origin of SIVagm90) and sabaeus monkeys from the Barbados. The virologic outcome in sabaeus and vervet AGM was surprisingly divergent. Inoculation of sabaeus AGM with SIVagm90 resulted in low and variable levels of primary viremia and low setpoints (< 100 to 10,000 copies/ml). In contrast, inoculation of vervet AGM resulted in high primary viremia and establishment of moderate plateau levels (10,000 to 100,000). Regardless of the extent of viremia, CD4+ T lymphocytes remained stable throughout infection of AGM with no significant relationship between viral load and CD4+ T cell loss. A large number of investigations have shown that CD8+ T cell responses are critical for the containment of AIDS viruses in humans and Asian nonhuman primates. We compared the phenotype of T cell subsets and magnitude of SIV-specific CD8+ T cell responses in chronically SIVagm-infected vervet AGM and SIVmac-infected rhesus monkeys (RM). In comparison to RM, vervet AGM exhibited lower signs of immune activation and associated proliferation of CD8+ T cells as detected by granzyme B, Ki-67, and PD-1 staining. SIV Gag and Env-specific immune responses were detectable at variable but slightly lower levels in vervet AGM by IFN-g ELISPOT assay and intracellular cytokine staining than observed in RM. These observations demonstrate that natural hosts like SIV-infected vervet AGM develop SIV-specific T cell responses, but the disease-free course of infection does not appear to depend on the generation of robust CD8+ T cell responses.